Methods for attenuating noise signals in a cement evaluation tool

ABSTRACT

Systems and methods for evaluating cement in the annulus of a wellbore are provided. In one embodiment, the cement may be evaluated using a casing arrival measurement sensor that measures casing arrival signals resulting from firing a signal from a cement bond logging acoustic source. Signals other than casing arrival signals may be attenuated using grooves between the transmitter and receiver in the drill collar.

FIELD

The subject disclosure relates to evaluating cement behind a casing of a wellbore. More specifically, the embodiments described herein relate to enhancing cement evaluation by attenuating vibrations within wellbore tools.

BACKGROUND

A wellbore drilled into a geological formation may be targeted to produce oil and/or gas from certain zones of the geological formation. To prevent zones from interacting with one another via the wellbore and to prevent fluids from undesired zones entering the wellbore, the wellbore may be completed by placing a cylindrical casing into the wellbore and cementing an annulus between the casing and wall of the wellbore. During cementing, cement is injected into the annulus formed between the cylindrical casing and the geological formation. When the cement properly sets, fluids from one zone of the geological formation are not able to pass through the wellbore to interact with another zone of the formation. This condition is referred to as “zonal isolation”. However, the cement may not set as planned and/or the quality of the cement may be less than expected.

A variety of acoustic tools may be used to verify that the cement is properly installed. For example, in a cement bond logging process, these acoustic tools may use pulsed acoustic waves as they are lowered through the wellbore to obtain acoustic cement evaluation data. The data collected by the acoustic tools may be used to determine whether or not the cement is likely to have set properly.

Unfortunately, these tools used in downhole cement bond logging activities often operate within extreme environments, such as high temperature, high pressure, and/or high shock environments. Because these tools are often delicate and highly sensitive to external vibration, the cement bond logging process has traditionally not been feasible in a logging-while-drilling (LWD) environment, where the cement bond logging tools are incorporated into the wellbore drill string. Instead, cement bond logging has traditionally been executed as an isolated process independent from a drilling process, as the drilling process may result in tool-damaging debris and external vibration data (e.g., drill collar vibration) that may alter the cement bond logging results.

For example, in a traditional scenario, a drill string may be placed in the wellbore, where a drill bit may be activated to drill to a certain depth (e.g., 1000's of feet). Once the drilling is complete, the drill string is then removed and the cement bond logging tools are independently dropped via a wireline service. Unfortunately, this independent process is oftentimes extremely time consuming and/or cost prohibitive because of the expense and timing of transporting, installing, and uninstalling an independent wireline service to a drilling rig or pad.

The challenge of designing logging while drilling (LWD) tools for cement bond logging is that signals propagating through a drill collar of the LWD tool contain no information on the bonding of cement-to-casing or cement-to-formation and, therefore, are considered noise signals.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

In one aspect, embodiments of the subject disclosure relate to a drilling tool with a drill collar. The drill collar has a transmitter configured to apply cement bond logging pulses to casing within a wellbore and a receiver configured to detect a casing arrival signal generated within the casing by the cement bond logging pulses. A plurality of grooves is positioned between the transmitter and the receiver configured to attenuate acoustic waves propagating through the drill collar.

In another aspect, embodiments of the subject disclosure relate to methods of inserting a drilling tool into a wellbore that is at least partially cased and cemented. The drilling tool comprises a transmitter, a receiver, and a drill collar. The transmitter is used to apply cement bond logging pulses to casing within the wellbore and the receiver is used to obtain an acoustic casing arrival signal generated within the casing by the cement bond logging pulses. Vibrations are attenuated within the drill collar between the transmitter and the receiver using a plurality of grooves that extend into the drill collar from an outer surface of the drill collar. A condition of cement is determined within a cement annulus of the wellbore using the acoustic casing arrival signal.

Further features and advantages of the subject disclosure will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:

FIG. 1 is a schematic diagram of a downhole tool that employs a cement bond logging tool for operation in a logging-while-drilling (LWD) environment, in accordance with an embodiment of the subject disclosure;

FIG. 2 is a graph of an E1 peak of a poorly bonded cemented and a well bonded cement;

FIGS. 3A and 3B depicts a design of grooves to attenuate wave propagation along a drill collar;

FIG. 4 depicts a pressure wave excited by the transmitter;

FIGS. 5A and 5B depicts a schematic of a typical wellbore with FIG. 5A depicting a well bonded cement to a casing and FIG. 5B depicting a gap filled with downhole fluid between casing and cement;

FIGS. 6A-6C depict a schematic for cement bonding evaluation using in FIG. 6A a first ideal tool, FIG. 6B a LWD tool without grooves in a drill collar, and FIG. 6C a LWD tool with grooves;

FIGS. 7A-7C depicts E1 peaks obtained for the tools in FIGS. 6A-6C; FIG. 7A depicts an E1 peak obtained by the first ideal tool for bonded and unbonded cement; FIG. 7B depicts an E1 peak obtained by the LWD tool without grooves for a bonded and unbonded cement; and FIG. 7C depicts an E1 peak obtained by the LWD tool with grooves for a bonded and unbonded cement;

FIGS. 8A and 8B depict a comparison between E1 peaks obtained by the first ideal tool, LWD tool without grooves and a grooved LWD tool for cement and casing which is bonded (FIG. 8A) and unbonded (FIG. 8B); and

FIG. 9 depicts a method of the subject disclosure.

DETAILED DESCRIPTION

The particulars shown herein are by way of example and for purposes of illustrative discussion of the examples of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements.

This disclosure relates to vibration attenuation for cement bond logging (CBL) tools in a logging-while-drilling (LWD) environment. Drilling a wellbore for an oil and/or gas well involves using a drill string that grinds into a rock formation. A cement bond logging tool with vibration attenuation capabilities may be included in the drill string, such that the cement bond logging process occurs in a LWD environment (e.g., an environment where the drill string is disposed in the wellbore). For example, when disposed in a drill string, CBL tools may be susceptible to inaccuracies based upon vibration of surrounding components. Accordingly, vibration attenuation capabilities are added to the CBL tools, as will be discussed in more detail below.

In general, a wellbore is drilled by rotating a drill string within the wellbore. The drill string includes a drill bit and a drill collar. As the drill string rotates, the drill bit also rotates and grinds away rock to drill deeper into the formation. In cases where the cement bond logging tool is included in the drill string as part of the drill collar, during operation of the drill string, the cement bond logging tool may be exposed to harsh environmental conditions, such as high pressure. Furthermore, even when the drill bit is not in operation, other equipment operation (e.g. operation of the CBL tool) may result in external vibration of components in the drill string (e.g., drill collar vibration). This external vibration may negatively impact accuracy of the cement bond logging results. For example, an acoustic transmission generated to excite a casing may also excite surrounding equipment (e.g., the drill collar). If not corrected, this excitation may result in less accurate cement bond logging results because signals resulting from the excitation of the surrounding environment may be erroneously interpreted as signals resulting from the excitement of the casing.

Accordingly, the cement bond logging tool described in this disclosure may include capabilities to attenuate the negative impact of these external vibrations on the cement bond logging results. Moreover, these attenuation capabilities may enable the cement bond logging tool to operate in a LWD environment, resulting in increased monetary cost and time efficiencies.

A drilling tool 10, shown in FIG. 1, includes a drill string 12 used to drill a wellbore 14 into a rock formation 16. A drill collar 18 of the drill string 12 encloses the various components of the drill string 12. Drilling fluid 20 from a reservoir 22 at a surface 24 may be driven into the drill string 12 by a pump 26. The hydraulic power of the drilling fluid 20 causes a drill bit 28 to rotate, cutting into the rock formation 16. The cuttings from the rock formation 16 and the returning drilling fluid 20 exit the drill string 12 through a space 30. The drilling fluid 20 thereafter may be recycled and pumped, once again, into the drill string 12.

A casing (e.g. cylindrical casing 32) may be placed into the wellbore 14 to prevent zones from interacting with one another and to prevent fluids from undesired zones entering the wellbore 14. To ensure the integrity of the casing 32, a cement annulus 34 may be formed by injecting cement into the annulus between the casing 32 and the rock formation 16.

A variety of information relating to the rock formation 16 and/or the state of drilling of the wellbore 14 may be gathered while the drill string 12 drills or remains inserted in the wellbore 14. For instance, a LWD tool may measure the physical properties of the rock formation 16 or cement annulus 34, such as density, porosity and resistivity. For example, in the illustrated embodiment, a cement bond logging tool (CBL) 36 is included in the drill collar 18, such that cement bond logging of the cement annulus 34 may be obtained.

As seen in FIG. 1, the drill string 12 is generally aligned along a longitudinal z-axis. Components of the drill string 12 may be located within the drill string at various radial distances from the z-axis, as illustrated by a radial r-axis. For example, a CBL tool 36 may be aligned with a portion of the cement annulus 34 with which cement bond logging is desired.

The CBL tool 36 may obtain acoustic measurements relating to the presence of solids or liquids behind the casing 32. For instance, the CBL tool may obtain an indication of the degree of solidity of the cement annulus 34 at a location where the CBL tool 36 is aligned. In some instances, the CBL tool 36 may obtain measurements as the drill string 12 is “tripping down” the wellbore 14 (e.g., as the CBL tool 36 passes by a segment of the well that was recently cemented, prior to the drilling of a next segment). The CBL tool 36 provides such measurements to surface equipment (e.g. using mud pulse telemetry or wired drill pipe). The surface equipment may include a data processing system 42 that includes a processor 44, memory 46, storage 48, and/or a display 50. In other examples, the data may be processed by a similar data processing system 42 at any other suitable location. The data processing system 42 may collect the data and determine one or more indices and indicators that, as discussed further below, may objectively indicate the presence or absence of properly installed cement. In this way, the data from the CBL tool 36 may be used to determine whether the cement annulus 34 has been installed as expected.

The CBL tool 36 may transmit (e.g. via a monopole transmitter) a signal to excite the casing 32. Upon excitation of the casing 32, the signal propagates into the casing 32, cement annulus 34 and rock formation 16. Resultant reflective signals, described herein as casing arrival signals are propagated back to a sensor of the CBL tool 36. These arrival signals may vary depending on the state (e.g., liquid or solid) of the cement annulus 34 behind the casing 32. For example, the amplitude of the arrival signals may be greater with increased annulus liquidity. The arrival signals are received and analyzed, such that a determination may be made as to the state of the cement annulus 34.

The CBL tool 36 may be protected from mud and debris travelling along the annulus 34, by installing shielding materials near the transmitters and/or sensors of the CBL tool 36. The transmission used to excite the casing 32 may also excite nearby drilling components, such as the drill collar 18. The excitation of the nearby drilling components (e.g. the drill collar 18), if not controlled, may negatively impact the determination of the state of the cement annulus 34, because these external signals (e.g., vibrations) may erroneously be factored into the data analysis as casing arrival signals.

A typical CBL tool is equipped with a transmitter which excites the acoustic pressure in well fluid, typically in the frequency of 20 kHz-25 kHz. A pressure wave is excited in the casing and some of the pressure wave energy is reflected at the interface (casing, cement, and formation) and some energy is transmitted across the interface. The amount of reflected and transmitted energies depends upon the acoustic impedance of the two materials at the interface.

In a poorly cemented zone, the casing plate extensional mode excited by the source loses little energy while propagating along the casing due to the very high acoustic impedance contrast between steel and fluid. When the casing is well cemented, part of the casing extensional mode energy enters into the cement and the formation through good coupling of the interface. Therefore, the attenuation of the casing extensional mode is very low in the poorly cemented zone and very high in the well cemented zone. The attenuation is affected by many factors such as cement properties, casing-wall thickness, and mud properties. Nevertheless, these properties are more or less the same over a certain interval of a well and differences in attenuation of the casing extensional mode can, therefore, be attributed to the presence of cement channels or the bonding conditions between the casing and cement. This relation is used to quantitatively evaluate the cement. When the cement is well bonded to the casing, the casing extensional mode is very small. To reliably measure the casing signal, cement bond logging (CBL) has historically been conducted at a spacing of 3 ft between the transmitter and receiver.

Acoustic waves propagate through materials in various modes. The extensional mode is the fastest propagating mode within the casing. The wavefront of the extensional mode is used for evaluating casing-to-cement bonding. In particular, amplitude of a first extensional mode (E1) within the casing is large in a poor-bonding interval and small in a good-bonding interval.

Attenuation of the extensional mode changes as a function of compressional acoustic impedance of cement, casing thickness and azimuthal cement bonding. A schematic plot on different amplitudes of the E1 mode is shown in FIG. 2 and from the amplitude of E1 mode, the quality of cement bonding can be estimated.

In an embodiment, uniformly distributed grooves are used to attenuate noise signals propagating along the drill collar. The grooves cause acoustic wave interference in the drill collar at a chosen frequency range so as to attenuate the acoustic energy propagating along the tool at the chosen frequency. Through numerical simulations, it has been shown that an LWD tool with this design can effectively detect a microannulus in cement-to-casing or cement-to-formation interfaces.

In a cement bond logging measurement, the transmitter generates acoustic energy through pressurizing the fluid. As discussed above, part of these energies propagate along the casing and are refracted back. The corresponding acoustic signal is useful for quantifying the bonding of cement. Part of the energy generates waves propagating through the shortest path between the transmitter and receiver which is typically through the tool. The corresponding acoustic signal, however, has no information on cement bonding and is considered noise. The noise signal in a wireline tool is insignificant because the space between the transmitter and receiver is mainly filled with fluid. However, this design cannot be applied to LWD cement bond logging tools because the transmitter-receiver space cannot be filled with fluid due to the strength requirements for LWD tools.

FIG. 3A depicts an LWD tool that includes a plurality of grooves 311 machined into the drill collar 315 between the transmitter 309 and the receiver 313. The LWD tool is positioned in a cased 305 wellbore with cement 303 between the casing 305 and the rock formation 301. Similar to FIG. 1, drilling fluid 307 may be driven into the drill string. The transmitter 309 and receiver 313 are embedded into a drill collar 315. In one embodiment, the transmitter and receiver are comprised of multiple ring elements extending around the circumference of the drill collar. In an embodiment, the ring transmitter has a fiberglass frame with many individual piezoelectric elements affixed to that frame and uniformly distributed around the circumference. The receivers also typically use small piezoelectric elements for the sensing, with multiple elements uniformly distributed around the circumference. In an embodiment without the grooves, the metal between the transmitter 309 and receiver 313 becomes a short-path for acoustic waves generated by the transmitter 309. The E1 peak through that wave arrives earlier than the E1 peak through the casing 305 which can reduce the sensitivity of the cement bond logging measurement. The plurality of grooves 311 attenuate these noise signals propagating along the drill collar 315. In non-limiting examples, heavy oil, drilling fluid and fiber glass, may be used to fill the grooves and thus attenuate the noise signal. Other materials contemplated include materials which have a slow-wave-speed (e.g. soft and dense materials). In other embodiments, the grooves do not contain material (e.g., filled by a gas or a vacuum). The collar arrival signal (i.e., the arrival signal resulting from excitation of the drill collar 351) is attenuated by using appropriately designed grooves on the drill collar 315 which can attenuate the drill collar signals.

FIG. 3B depicts a plurality of grooves 311 of the subject disclosure which have a uniform, width (W), height (H) and edge to edge distance (D) between the grooves. As shown in FIG. 3B, the plurality of grooves 311 are cut inward from an outer surface of the drill collar 315. In general, the width (W) and the edge to edge distance (D) between the grooves are determined using a number of factors. The factors include one or more of: (i) the material of the drill collar, (ii) the inner and outer diameters of the drill collar, and (iii) the frequency of the transmitter used. The wavelength for the transmitter, in general, is estimated within certain ranges, in non-limiting examples the transmitter frequency is 20 Khz, and the optimized groove distance is estimated using numerical simulations. In a non-limiting example, the width of the grooves (W) and the edge-to-edge distance (D) between grooves is set to one quarter of the wavelength of a first extensional mode propagating along the drill collar 315. This wavelength is also dependent on the material used for the drill collar 315. In non-limiting examples, the material of the drill collar is a non-magnetic stainless steel. In other examples, the material may be titanium or inconel. In examples, the width of the grooves (W) and the edge to edge distance (D) between grooves is between 50 mm and 100 mm. Attenuation of noise is increased by using grooves that are cut deeper into drill collar 315 but groove depth is increased only to a value which will not compromise tool strength requirements. In one example, a depth of 25 mm has been used for the groove depth. The groove depth will depend on the inner and outer diameter of the drill collar and the thickness of the collar remaining after placement of the grooves.

Numerical simulations are used to validate the design. The signal excited by the transmitter is the second derivative of Blackman-Harris window, a time series, with the frequency 20 kHZ, as shown in FIG. 4. The space between the transmitter and the receiver is less than or equal to 1 meter, similar to a typical wireline tool.

In an isotropic and linear elastic media, the velocity fields v and stress fields τ are governed by:

$\begin{matrix} {{{\rho \frac{\partial v}{\partial t}} = {\nabla\; {\cdot \tau}}}{{\frac{1}{\rho}\frac{\partial\tau}{\partial t}} = {{\left( {c_{P}^{2} - {2\; c_{S}^{2}}} \right)\left( {\nabla{\cdot v}} \right)I} + {c_{S}^{2}\left( {{\nabla v} + {v\nabla}} \right)}}}} & (1) \end{matrix}$

where ρ is the density, c_(P) and c_(S) are the speed of extensional and shear wave of the media, respectively. Note that c_(S)=0 for the liquid media. Typical coefficient values for downhole fluid, casing, cement and formation are specified in Table 1 below. The governing equations (1) are solved numerically using a finite difference method with the initial and boundary conditions.

TABLE 1 Material properties used for numerical simulations. downhole fluid casing cement rock ρ (kg/m³) 1000 7830 1496 2800 c_(P) (m/s) 1693 5650 2725 6773 c_(S) (m/s) 0 3170 1669 3586

FIGS. 5A and 5B depict a cased 501 wellbore with cement 503 between the casing 501 and the rock formation 505. Two cases for the bonding between cement and casing are analyzed: one bonded as depicted in FIG. 5A and the other un-bonded with a liquid layer 507 between the casing 501 and cement 503 as depicted in FIG. 5B. FIGS. 6A-6C depicts three types of tools which are considered in a cased 607 wellbore with cement 609 between the casing 607 and the rock formation 613. The first tool is an idealized tool, i.e., a transmitter 601 and receiver 603 are spaced 1 meter apart, with mud (615) between the source and receiver (i.e., a wireline tool), as illustrated in FIG. 6A. In this situation, the wave propagation along the shortest path between transmitter 601 and receiver 603 has an insignificant effect on the E1 peak measurement. Therefore, the idealized tool scenario is used as a reference. The second tool as depicted in FIG. 6B shows the transmitter 601 and receiver 603 embedded into a drill collar 611 without a plurality of grooves. This case is used to analyze noise signals propagating along the drill collar 611. The third tool is an embodiment of the subject disclosure that includes a receiver 601 and a transmitter 603 embedded in the drill collar 611 and separated by a plurality of grooves 605. The measurement from this third tool is close to that from tool one.

The signal received by receivers for the idealized tool, tool without grooves, and tool with grooves are shown in FIGS. 7A-7C, respectively. In the idealized tool, the E1 peaks arrive at 270 is for both the bonded 703 case and the unbonded 701 case. In addition, a clear contrast for the E1 peak is observed for these two cases (i.e., the E1 peak for the unbonded case is 4.8 times to the bonded case).

For the tool with no grooves, both E1 peaks arrive at 220 μs which indicates that the acoustic signal propagates through the drill collar rather than the casing. In addition, for both bonded and unbonded cases, the E1 peak is the same, which is due to the first arrived extensional mode in the tool which contains no information on the bonding between casing and cement.

For the tool with grooves, both E1 peaks arrive at 270 μs. This indicates that the wave propagation along the collar is attenuated. The contrast between the two peaks is 3.9 (slightly lower than the idealized tool).

FIGS. 8A and 8B compare the E1 peak obtained by the three different types of tools above for bonded and unbonded cases. The tool without grooves, the E1 peak contains no information on the bonding between the casing and cement. The tool with grooves, the E1 peak is very close to that obtained by the ideal tool.

FIG. 9 depicts a method of the subject disclosure. The method comprises in a first process 901, inserting a drilling tool into a wellbore that is at least partially cased and cemented. The inserted drilling tool has a transmitter, a receiver and a drill collar. In a second process 903, the transmitter is used to apply cement bond logging pulses to casing within the wellbore. In a third process 905, the receiver is used to obtain an acoustic casing arrival signal generated within the casing by the cement bond logging pulses. In a fourth process 907, vibrations are attenuated within the drill collar between the transmitter and the receiver using a plurality of grooves that extend into the drill collar from an outer surface of the drill collar. Finally, in a fifth process 911, a condition of cement is determined within a cement annulus of the wellbore using the acoustic casing arrival signal.

As explained herein, by placing the CBL tool 36 in the drill collar 18, the CBL tool 36 may be used in extreme environmental conditions, such as high temperature environments. By incorporating the vibration attenuation capabilities into the CBL tool 36, vibrations associated with the drill collar 18 may be attenuated, such that accurate cement bond logging results may be obtained, despite being disposed in a potentially highly excitable environment (e.g. a metallic drill collar).

Accordingly, in contrast to many traditional CBL processes that use an independent cement bond logging service, the CBL tool 36 described herein may be used in an environment where a drill string is operating in the wellbore. As may be appreciated, removing an independent CBL process may decrease time and monetary expenditures because the CBL process may occur as part of the drilling process.

Although only a few examples have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the examples without materially departing from this subject disclosure. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. 

What is claimed is:
 1. A drilling tool comprising: a drill collar that comprises: a transmitter configured to apply cement bond logging pulses to casing within a wellbore; a receiver configured to detect a casing arrival signal generated within the casing by the cement bond logging pulses; and a plurality of grooves positioned between the transmitter and the receiver, and configured to attenuate acoustic waves propagating through the drill collar, wherein at least one of the plurality of grooves extends from an outer surface of the drill collar.
 2. The drilling tool of claim 1, wherein the transmitter and the receiver are separated by a distance of less than or equal to 1 meter.
 3. The drilling tool of claim 1, wherein each groove comprises a width between 50 mm and 100 mm.
 4. The drilling tool of claim 1, wherein each groove comprises a width and the plurality of grooves have uniform widths.
 5. The drilling tool of claim 1, wherein the grooves are spaced apart by a distance between 50 mm and 100 mm.
 6. The drilling tool of claim 1, wherein the grooves are spaced apart by a uniform distance.
 7. The drilling tool of claim 1, wherein the cement bond logging pulses are applied at a frequency between 20 kHz and 25 kHz.
 8. The drilling tool of claim 1, further comprising: a data processing system configured to determine a condition of cement disposed within a cement annulus of the wellbore using the casing arrival signal.
 9. The drilling tool of claim 1, further comprising an acoustic damping material disposed in the grooves.
 10. The drilling tool of claim 9, wherein the acoustic damping material includes at least one of a heavy oil, a drilling fluid, and a fiber glass.
 11. The drilling tool of claim 1, where the drilling tool is a logging-while-drilling tool.
 12. A method comprising: inserting a drilling tool into a wellbore that is at least partially cased and cemented, wherein the drilling tool comprises a transmitter, a receiver, and a drill collar; using the transmitter to apply cement bond logging pulses to casing within the wellbore; using the receiver to obtain an acoustic casing arrival signal generated within the casing by the cement bond logging pulses; attenuating vibrations within the drill collar between the transmitter and the receiver using a plurality of grooves that extend into the drill collar from an outer surface of the drill collar; and determining a condition of cement within a cement annulus of the wellbore using the acoustic casing arrival signal.
 13. The method of claim 12, wherein the condition of cement comprises a cement bond defect within the cement annulus.
 14. The method of claim 13, wherein the cement bond defect is a microannulus.
 15. The method of claim 12, wherein each groove comprises a width and the method further comprises: determining the width of the grooves based on (i) a material of the drill collar, (ii) inner and outer diameters of the drill collar, and (iii) a frequency of the cement bond logging pulses applied by the transmitter.
 16. The method of claim 12, wherein the transmitter and the receiver are separated by a distance of less than or equal to 1 meter.
 17. The method of claim 12, wherein each groove comprises a width between 50 mm and 100 mm.
 18. The method of claim 12, wherein each groove comprises a width and the plurality of grooves have uniform widths.
 19. The method of claim 12, wherein the grooves are spaced apart by a uniform distance.
 20. The method of claim 12, wherein the cement bond logging pulses are applied at a frequency between 20 kHz and 25 kHz. 